Quirky solar cell sets new efficiency record

Researchers in the US have built a new type of solar cell that emits light as well as absorbs it, making it the most efficient single-junction device ever developed. The efficiency of their prototype cell allows it to convert 28.6% of the Sun's energy into electricity. This is a considerable increase from the previously recorded highest efficiency of 26.4%, which was achieved in 2010.

Scientists have known since 1961 that the absolute limit for the amount of energy that can be harvested from sunlight hitting a typical solar cell is about 33.5%. However, for almost five decades researchers have been unable to come close to achieving this theoretical efficiency. But now, Eli Yablonovitch and his graduate student Owen Miller from the University of California, Berkeley have designed and built a new type of solar cell that gets closer to that limit by mimicking the behaviour of a light-emitting diode. That is to the say the solar cell is highly capable of absorbing light as well as emitting it. In fact, it is the controlled emission of light that has boosted the efficiency.

The researchers have shown that the better a solar cell is at emitting photons, the higher its voltage is and the greater its efficiency. "[The result] is almost paradoxical and counterintuitive. It can be quite confusing to grasp at first," says Yablonovitch, as he tells physicsworld.com that he and his colleagues discovered the connection while trying to resolve the large gap between the theoretical and achieved limits for solar-cell efficiency.

Managing photons

The solution lay in a mathematical connection between absorption and emission of light – a phenomenon better understood as "photon management". Conventionally, photon management involves controlling the photons incident on a solar cell so that a photon ejects as many electrons as possible, thereby generating the maximum amount of electric current. "But there is another aspect to photon management, in that we manage not only the incident light, but also the emitted light. Emitted photons sometimes get 'lost' within the cell, so what we do is make sure those photons are emitted," explains Yablonovitch. In a conventional solar cell, photons from the Sun hit a semiconductor material, knocking electrons loose and allowing them to flow freely. But this process can also generate new photons, in a process known as "luminescent emission". As there is a fundamental thermodynamic link between absorption and emission, designing solar cells to emit light causes an increase in the voltage produced by the device.

The researchers' novel concept has been put into practice by a company called Alta Devices, which was co-founded by Yablonovitch and California Institute of Technology physicist Harry Atwater in 2007. The firm was set up specifically to produce economic and high-energy solar cells. The new prototype solar cell is made of gallium arsenide, a material often used to make solar cells for satellites. The result is a device that operates at 28.6% efficiency.

First to put into practice

While the theory of luminescent emission causing an increase in voltage has been known for a while, it has never been put into practice. "It is somewhat puzzling why it has never been used in the field of solar-cell development until now. But a lack of certain requirements might explain that," says Yablonovitch. He goes on to say that solar cells are "grown" on substrates that are generally of poor quality and act as "sinks" for the emitted luminescent photons, which are then lost. The new cell made by Alta Devices is separated from the substrate, which delivers a much better performance. "In fact, we separate the substrates on which the cells are grown and then re-use them. This not only helps with efficiency, but it also brings the cost of producing our cells down, and so it is a key factor," says Yablonovitch. He explains that the cells are still as thin (1 µm) as traditional cells and so people are genuinely shocked to know the devices have been developed cheaply using gallium arsenide. Alta Devices is already producing the cells on an industrial scale, with samples being shipped to customers.

Yablonovitch says he hopes researchers will be able to use this technique to achieve efficiencies close to 30% in the coming years. And given that the work applies to all types of solar cells, the findings have implications throughout the field.

The team will present its findings at the Conference on Lasers and Electro Optics to be held in early May in California in the US.

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20 comments

Good idea and the work based on it.

Good step forwards based on the fundamental idea that there is a mathematical connection between the absorption and emission of light. This implies that higher the emmission of light (as photons, higher the voltage developed in the cell allowing it to have a higher conversion efficiency.This idea led the team to build a solar cell (1μm thick as the other cells) of gallium arsenide which has an efficieny of 28.5 % compared to the Shockly-Queisser maximum theoretical limit of 33.5 %. Could one of the team of this work, spell out or give a reference, where this idea is presented algerically? One will be thankful for this effort.

headline is not strictly true

This article should be amended to make clear that the Alta cell is not "the most efficient solar cell ever built", but the most efficient non-concentrating, single-junction solar cell ever built. There is an important difference. Tandem-stack and multi-junction solar cells exceed 30% efficiency, and can exceed 40% with concentration. The "standard reference" for this is the series titled "Solar cell efficiency tables" published annually in the journal Progress in Photovoltaics: Research and Applications. The latest (number 39) was published in January this year and lists several devices having higher efficiencies than the Alta cell.

This is not to belittle the remarkable achievement of Yablonovitch, Miller, and all at Alta. They should of course be congratulated for their excellent and original work. But Physics World should report science and technology news accurately, and the fact that you have not discriminated between single-junction cells and multi-junction cells is a failing in this duty. It could lead to misunderstanding among some readers, who may remember stories such as this one from 2009 reporting on a 43% efficient cell. That is why I point it out, and I hope you will change the article to make this important technological distinction plain and clear.

Well Done!!

Congratulations for the new results,

its will make a revoloution in solar cell;s application and industry. I always believed as a physician that Si is not a proper material for photovoltaics as it's indirect semiconductor. So, now we have GaAs (a direct semic.) and cheaper cell. Vivaaaaa

Mr Fill, i cannot agree more with you on the need to distinguish between single and multi-junctions cells. The article may mislead readers especially those who dont closely follow the solar cells progress...

Mr Fill, i cannot agree more with you on the need to distinguish between single and multi-junctions cells. The article may mislead readers especially those who dont closely follow the solar cells progress...

I would really appreciate if somebody explained the difference at least here in comments. If this efficiency is so important 'with single junction' and 'without concentrator' does it mean this same cell can be somehow reconfigured for even higher efficiency if we use multiple junctions and concentrator with it, whatever they are?

This article should be amended to make clear that the Alta cell is not "the most efficient solar cell ever built", but the most efficient non-concentrating, single-junction solar cell ever built. There is an important difference. Tandem-stack and multi-junction solar cells exceed 30% efficiency, and can exceed 40% with concentration. The "standard reference" for this is the series titled "Solar cell efficiency tables" published annually in the journal Progress in Photovoltaics: Research and Applications. The latest (number 39) was published in January this year and lists several devices having higher efficiencies than the Alta cell.

This is not to belittle the remarkable achievement of Yablonovitch, Miller, and all at Alta. They should of course be congratulated for their excellent and original work. But Physics World should report science and technology news accurately, and the fact that you have not discriminated between single-junction cells and multi-junction cells is a failing in this duty. It could lead to misunderstanding among some readers, who may remember stories such as this one from 2009 reporting on a 43% efficient cell. That is why I point it out, and I hope you will change the article to make this important technological distinction plain and clear.

This article should be amended to make clear that the Alta cell is not "the most efficient solar cell ever built", but the most efficient non-concentrating, single-junction solar cell ever built. There is an important difference. Tandem-stack and multi-junction solar cells exceed 30% efficiency, and can exceed 40% with concentration. The "standard reference" for this is the series titled "Solar cell efficiency tables" published annually in the journal Progress in Photovoltaics: Research and Applications. The latest (number 39) was published in January this year and lists several devices having higher efficiencies than the Alta cell.

This is not to belittle the remarkable achievement of Yablonovitch, Miller, and all at Alta. They should of course be congratulated for their excellent and original work. But Physics World should report science and technology news accurately, and the fact that you have not discriminated between single-junction cells and multi-junction cells is a failing in this duty. It could lead to misunderstanding among some readers, who may remember stories such as this one from 2009 reporting on a 43% efficient cell. That is why I point it out, and I hope you will change the article to make this important technological distinction plain and clear.

Resonance effect

For me it looks like a resonnance effect,There it is typical that the voltage raises when you transmit more.For some info on the concept look at this:amasci.com…tesceive.htmlthey kind of do this to convert some heat in thermonuclear devices directly to electricity with relativ high efficiency with electron emissiv matterials an a resonant coupler since at least the 1980.

What about the reflected light?

Apart from the absorbed and internally emitted light, the solar cell must also reflect some of the originally impinging sunlight, which would be wasted. The cells in the photo seem rather matte but still.. I am not sure what the references to tandem and stacked tiles in the discussion above refer to- But once I had the idea of having a large solar cell tiling on a parabolic (not flat) base so that any reflected light can be collected by a small additional cell at the focus point.

Efficieny of single-junction solar cell

Vladimir, this 28.6% conversion efficiecy of the single-junction cell is the amount of the solar EM-spectrum energy that produces elctrons for the current and the remaining about 71% is lost in different ways including the reflection. The trick here seems to be that maximising (after the absorption of radiation) the emission of photons maximises the voltage of the cell implying a higher efficiency. Unfortunately, I do not have this mathematical relation to present here. May be a member of the team involved in this work, could provide this relation or a good reference to it. May be they have given all this in their paper, but I have not been able to reach it till now. Moreover, the conversion efficiency of these single-junction solar cells is slowly approaching the Shockley-Queisser theoretical limit of 33.5%.

Apart from the absorbed and internally emitted light, the solar cell must also reflect some of the originally impinging sunlight, which would be wasted. The cells in the photo seem rather matte but still.. I am not sure what the references to tandem and stacked tiles in the discussion above refer to- But once I had the idea of having a large solar cell tiling on a parabolic (not flat) base so that any reflected light can be collected by a small additional cell at the focus point.

Are the panels clean? What happens when they age? Here's a quote from wikipedia "The toxicological properties of gallium arsenide have not been thoroughly investigated. On one hand, due to its arsenic content, it is considered highly toxic and carcinogenic. On the other hand, the crystal is stable enough that ingested pieces may be passed with negligible absorption by the body. When ground into very fine particles, such as in wafer-polishing processes, the high surface area enables more reaction with water releasing some arsine and/or dissolved arsenic. The environment, health and safety aspects of gallium arsenide sources (such as trimethylgallium and arsine) and industrial hygiene monitoring studies of metalorganic precursors have been reported.[14] California lists gallium arsenide as a carcinogen."

The reflected sunlight

Asghar - the 'lost' sunlight I was thinking of being recollected is the small amount of radiation (depending on angle, surface quality, material etc.) that is not absorbed nor enters the cells but is reflected back into space by the outer plastic or glass cover. The efficiency discussions are presumably about the portion of the radiation that does make it into the cell.Quote:

Vladimir, this 28.6% conversion efficiecy of the single-junction cell is the amount of the solar EM-spectrum energy that produces elctrons for the current and the remaining about 71% is lost in different ways including the reflection. The trick here seems to be that maximising (after the absorption of radiation) the emission of photons maximises the voltage of the cell implying a higher efficiency. Unfortunately, I do not have this mathematical relation to present here. May be a member of the team involved in this work, could provide this relation or a good reference to it. May be they have given all this in their paper, but I have not been able to reach it till now. Moreover, the conversion efficiency of these single-junction solar cells is slowly approaching the Shockley-Queisser theoretical limit of 33.5%.

Apart from the absorbed and internally emitted light, the solar cell must also reflect some of the originally impinging sunlight, which would be wasted. The cells in the photo seem rather matte but still.. I am not sure what the references to tandem and stacked tiles in the discussion above refer to- But once I had the idea of having a large solar cell tiling on a parabolic (not flat) base so that any reflected light can be collected by a small additional cell at the focus point.

Normalised EM-spectrum

Vladimir, The solar power(energy/sec) incident on Earth is ~ 1kW/ m^2. This value helps to normalise the solar EM-spectrum. This is used to determine the efficiecy of a solar cell. Moreover, as technical point, during the construction, one has to minimise the loss due to this reflection.

Asghar - the 'lost' sunlight I was thinking of being recollected is the small amount of radiation (depending on angle, surface quality, material etc.) that is not absorbed nor enters the cells but is reflected back into space by the outer plastic or glass cover. The efficiency discussions are presumably about the portion of the radiation that does make it into the cell.Quote:

Vladimir, this 28.6% conversion efficiecy of the single-junction cell is the amount of the solar EM-spectrum energy that produces elctrons for the current and the remaining about 71% is lost in different ways including the reflection. The trick here seems to be that maximising (after the absorption of radiation) the emission of photons maximises the voltage of the cell implying a higher efficiency. Unfortunately, I do not have this mathematical relation to present here. May be a member of the team involved in this work, could provide this relation or a good reference to it. May be they have given all this in their paper, but I have not been able to reach it till now. Moreover, the conversion efficiency of these single-junction solar cells is slowly approaching the Shockley-Queisser theoretical limit of 33.5%.

Apart from the absorbed and internally emitted light, the solar cell must also reflect some of the originally impinging sunlight, which would be wasted. The cells in the photo seem rather matte but still.. I am not sure what the references to tandem and stacked tiles in the discussion above refer to- But once I had the idea of having a large solar cell tiling on a parabolic (not flat) base so that any reflected light can be collected by a small additional cell at the focus point.

Vladimir, The solar power(energy/sec) incident on Earth is ~ 1kW/ m^2. This value helps to normalise the solar EM-spectrum. This is used to determine the efficiecy of a solar cell. Moreover, as technical point, during the construction, one has to minimise the loss due to this reflection.